Little is known about the eVects of cover crops on soil properties in organic orchards. To Wll this gap, this work aimed at examining the eVects of several cover crops on soil fertility, nitrate dynamics, populations of nematodes and tree performance in an organic orchard of apple cv. Royal Gala/EM 26 planted in 1994 at 4 £ 2 m. In 1999 the following treatments were randomly applied to the inter-row spaces of 20 tree rows each: permanent cover of alfalfa (Medicago sativa L.) plus fescue (Festuca arundinacea Schribn.), Alfalfa/fescue (AF); permanent cover of strawberry clover (Trifolium fragiferum L.), Strawberry clover (SC); seeding of common vetch (Vicia sativa L.), Vetch (V); and Control (C) (natural vegetation of grasses and legumes with the soil disked twice in late winter, which is the traditional management system used by growers in this region). The cover crops were mowed 3 or 4 times during the growing season and the clippings were left on the ground for decomposition. The trial was conducted in the northern Patagonia region of Argentina in a sandy loam soil with a pH of 7.6 and initial organic matter content of 1.5%. A commercially available certiWed organic fertilizer containing 5% total N, 2% total P and 4% total K was added annually at a rate of 1.0 kg per tree in equal amounts to each treatment in a radius of 0.4 m around the trunk. After 6 years, soil organic matter in the top soil was 31, 27.9, 23 and 18.6 g kg ¡1 for SC, AF, V and C treatments, respectively. Total soil N followed the same trend. Apple leaf N declined steadily in all treatments especially in AF and C from year 3 to 5 with values below 1.8% and it was therefore necessary to increase the rate of organic fertilizer. Nitrate concentrations remained under 7.5 mg kg ¡1 during the winter months in all treatments and increased in spring and summer. A sharp increase in soil nitrate, up to 100 ppm, was observed in late spring in V due to rapid biomass decomposition after maturation of the common vetch. Populations of nematodes especially bacterivores and herbivores increased from September (late winter) to March (fall) in the V treatment. SigniWcant diVerences were observed when we compared the eVects of treatments on the entire nematode population. Tree growth as measured by trunk cross sectional area and canopy volume was signiWcantly lower in the C treatment.

A B S T R A C TThe aim of this work was to evaluate the vegetativeproductive balance of Vitis vinifera L. cvs. Merlot, Malbec and Cabernet Sauvignon on Aridisols with hardened layers in the Alto Valle of Río Negro, Argentina. The study was carried out in 15.5 ha of vineyards during 2002-2003 on five soil groups characterized by: type, thickness and depth of the horizons, texture and resistance to penetration. In each sampling site three plants were evaluated. Production, leaf area, pruning material, trunk diameter, the Ravaz Index, the leaf area/yield ratio and the leaf area index were determined. A variance analysis and means comparison were made. The production level, the leaf area and the pruning material of the three cultivars were reduced (P < 0.05) when the soils presented extremely hard horizons (resistance to penetration greater than 3 MPa) at a depth of 35 cm or less and with 30 cm or more of thickness, indicating a low site potential. The Ravaz Index and the leaf area/yield ratio didn't show differences (P < 0.05) among soil groups in any cultivar. In soils with restricted properties, but with an adequate growth-yield balance, smaller vine spacing and avoiding gaps between vines can be used to increase production.

In the Upper Rio Negro Valley, northern Patagonia (Argentina), there is a large variability of soils and the success of a fruit plantation depends mostly on soil characteristics. The main objective of this work was to determine the relationships between yield and fruit internal maturity at harvest of pear trees (Pyrus communis L.) grown in soils with different properties. The soil around each of 30 trees was morphologically characterized by measuring physical, physic-chemical, and chemical variables. At harvest time, total and commercial yields were measured, and maturity was determined by measuring flesh firmness, soluble solids, and titratable acidity. According to edaphic variables, soils were classified into five groups. Yields presented significant differences among such groups and growing seasons. The distribution of commercial sizes was similar in soils with fine texture, and high K content (897 and 663 mg kg -1 ) and cation-exchange capacity. Smaller fruits predominated in soil groups with high salinity (6.1 dS m -1 in surface and 10.8 dS m -1 in subsurface horizons) and shallow depth. The soil group with the highest salinity presented fruits with 11% more titratable acidity compared to all other groups. The soil group with the smallest depth presented fruits with high levels of flesh firmness (11.18 kg cm -2 ) and soluble solids (14.8%). The selected set of edaphic variables allowed us to differentiate groups of soils and analyze its influence in pear tree yield, commercial sizes, and fruit internal maturity.

A nematode community was used as a bioindicator of changes in agroecosystems caused by fertilization. The effect of applying nitrogen (N) fertilizer on a soil nematode community structure was studied in a soil which had not been fertilized for six years in the Rio Negro Valley, Argentina. Treatments were: i) 100 kg N ha-1 (N100); ii) 200 kg N ha-1 (N200); in each case 50% of the dosage was applied at the time of petal fall and 50% at fruit harvest in 2004/2005, 2005/2006, 2006/2007 growing seasons; and iii) control with no fertilizer (N0). Soil samples were collected in the 0 to 30 cm soil layer in October, November, April and July in each growing season. The number of bacterivores increased in N200 compared to N0. Cephalobidae were present in greater numbers in N200 than in N100 and N0. Predator abundance was lower under N200, after the first N application in each growing season. The ratio of fungivores to bacterivores (F/B) was 0.21, 0.3 and 0.41 in N200, N100 and N0, respectively. N200 resulted in a community with a lower maturity index (MI) than N0. Structure index (SI) was lower in N200 than N100 and N0. The enrichment index (EI) was less sensitive at detecting fertilizer effects. In November and April, soil nitrate concentrations were higher in N200 than in N100 and N0. Soil nitrate concentration was positively correlated with EI and negatively with MI, SI and F/B. N200 affected the trophic structure of the nematode community consistent with a less stable soil system.

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Nitrogen utilization of different fertilization regimes in lettuce (Lactuca sativa L.) and its efect on biological properties of the soil on Neuquén, Argentina.

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